1. Field of the Invention
The present invention relates to a vibrator, which is used in an optical deflector, an acceleration sensor, an angular velocity sensor, or the like, the vibrator including a movable plate, which is supported from both sides. The present invention also relates to a manufacturing method for manufacturing such a vibrator.
2. Description of the Related Art
The idea of manufacturing optical deflectors by silicon micro-machining is reported by K. Petersen in 1980. In recent years, not only optical deflectors but also acceleration sensors and angular velocity sensors are being developed by use of the same technology. These types of apparatus use a vibrator that comprises a movable plate supported by an elastic member and being capable of vibrating. When applied to an optical deflector, the movable plate is provided with a reflecting plate. To provide a great amplitude of vibration based on a small amount of power, it is desirable that the movable plate be vibrated by utilization of resonance. When applied to an acceleration sensor or an angular velocity sensor, the movable plate is used as a detecting member for detecting the acceleration or angular velocity.
In principle, in this type of vibrator, the resonance frequency is determined by the stiffness of the elastic member and the inertial property of the movable plate. To actuate the vibrator by utilization of resonance, a driving signal having the same frequency as the resonance frequency is applied, but the actual resonance frequency varies due to various factors. One of the major factors is the temperature, and the stiffness of the elastic member varies in accordance with a change in the temperature (in general, the stiffness decreases in accordance with an increase in the temperature), resulting in a change in the resonance frequency.
There are some vibrators that must be driven on a constant frequency, and the resonance frequency of such vibrators must be kept constant. This requirement arises depending upon the purposes for which the vibrators are used. To satisfy this requirement, several techniques for realizing a constant resonance frequency of a vibrator have been proposed to date
Jpn. Pat. Appln. KOKAI Publication No. 9-197334 discloses a vibrator wherein an elastic member is provided with a resistance heating type of heater and the temperature of that elastic member is controlled. In this vibrator, the heater is so controlled as to keep the temperature of the elastic member constant despite changes in the environmental temperature, and the resonance frequency is stabilized thereby.
Due to the additional use of the heater, the manufacturing cost of the vibrator of Jpn. Pat. Appln. KOKAI Publication No. 9-197334 is high. In addition, since the heater is continuously supplied with current, the power consumption inevitably increases. Furthermore, since the temperature of the elastic member is controlled based on the heat generated by the heater, effective measures cannot be taken to cope with an increase in the environmental temperature.
U.S. Pat. No. 5,557,444 discloses a vibrator provided with a temperature-compensating rod. The temperature-compensating rod is independent of the vibrator and cancels the temperature-dependent stiffness change of the elastic member by utilization of the difference in thermal expansion coefficients. To be more specific, the temperature-compensating rod changes its length in accordance with a temperature in the environmental temperature, thereby providing the elastic member with stress that serves to cancel the stiffness variation of the elastic member. Hence, the resonance frequency is stabilized.
Due to the additional use of the temperature compensating rod, the manufacturing cost of the vibrator of U.S. Pat. No. 5,557,444 is inevitably high. According to this U.S. patent, the change in the length of the temperature compensating rod stresses the base member that holds the vibrator, and the deformation of the stressed base member stresses the elastic member of the vibrator. In general, the base member is thought to be very rigid, and it is questionable whether the elastic member of the U.S. patent produces a desired stress.